How do you get rid of the sewage smell after a backup?

Odor is a symptom, not the problem. If a backup smell lingers after cleanup, it usually means porous material (carpet pad, drywall, subfloor) still holds contamination, or the structure was never fully dried and mold has started.

One easy cause worth checking: a floor drain or unused fixture whose P-trap dried out lets sewer gas rise straight into the room. Refilling the trap can solve a mysterious sewer smell that has nothing to do with the original backup.